Acetylcholine (ACh) is a key neurotransmitter that modulates neuronal function in the peripheral nervous system (PNS) and central nervous system (CNS). ACh mediates its actions via two families of receptors, termed the muscarinic ACh receptors (mAChRs) and the nicotinic ACh receptors (nAChRs). A large body of evidence suggests that basal forebrain cholinergic neurons and basalo-cortical cholinergic pathways are selectively vulnerable to degeneration in Alzheimer's disease. It has therefore been hypothesized that cholinergic hypofunction contributes to the cognitive deficits of patients suffering from Alzheimer's disease. Consequently, acetylcholinesterase inhibitors, which inhibit ACh hydrolysis and potentiate cholinergic signaling have been demonstrated to not only provide improvements in cognitive symptoms associated with Alzheimer's disease, but also show efficacy in treating the psychiatric symptoms. Acetylcholinesterase inhibitors, however, have not been shown to change the underlying disease pathology.
Another potential pharmacotherapeutic target to counteract cholinergic hypofunction is the activation of muscarinic acetylcholine receptors (mAChRs). Muscarinic acetylcholine receptors are members of the G protein-coupled receptor superfamily which mediate the actions of the neurotransmitter acetylcholine in both the central and peripheral nervous system. Muscarinic acetylcholine receptors are prevalent throughout the body and five distinct muscarinic receptors (M1-M5) have been identified in mammals. The muscarinic receptors are known to contain one or more allosteric sites which may alter the affinity with which muscarinic ligands bind to the primary binding or orthosteric sites. In the central nervous system, muscarinic receptors are involved in cognitive, behavior, sensory, motor and autonomic functions. The M4 muscarinic acetylcholine receptor is predominantly expressed in the striatum, but also in the hippocampus and cortex.
Muscarinic receptors in the central nervous system play a critical role in mediating higher cognitive processing and control of dopamine release. Administration of nonselective muscarinic antagonists can induce cognitive deficits and psychosis in humans suggesting that mAChR activation may provide pro-cognitive and antipsychotic efficacy. Accordingly, several mAChR agonists have been developed and entered clinical studies for the treatment of cognitive and psychiatric symptoms associated with Alzheimer's and neuropsychiatric diseases such as schizophrenia. (Carruthers, Neuroscience & Biobehavioral Rev., 2015, 55: 393-402; Jones, et al. Neuropsychopharmacology, 2012, 37: 16-42). One of these, the M1/M4 preferring mAChR agonist xanomeline was assessed in patients with Alzheimer's disease, and while showing a trend for improving cognitive deficits, did produce robust and dose-dependent reductions in hallucinations, delusions, vocal outbursts, and other behavioral disturbances in these patients. A subsequent study in patients with schizophrenia demonstrated that xanomeline produced robust improvements in positive, negative and cognitive symptoms. (Bodick, et al., Arch Neurol. 1997; 54: 465-73). Xanomeline, in addition to other mAChR agonists have been demonstrated to produce robust antipsychotic-like effects in a number of preclinical paradigms. For instance, xanomeline, reverses a number of dopamine driven behaviors, including amphetamine induced locomotion in rats, apomorphine induced climbing in mice, dopamine agonist driven turning in unilateral 6-OH-DA lesioned rats and amphetamine induced motor unrest in monkeys (without EPS liability). It also has been shown to inhibit A10, but not A9, dopamine cell firing and conditioned avoidance and induces c-fos expression in prefrontal cortex and nucleus accumbens, but not in striatum in rats. These data are all suggestive of an atypical antipsychotic-like profile. Subsequent studies with M4 knockout mice have demonstrated that the antipsychotic-like effects of xanomeline are mediated by the M4 receptor. Despite these promising clinical and preclinical effects, xanomeline, like other muscarinic agonists, ultimately failed in clinical development due to lack of adequate receptor subtype selectivity resulting in dose-limiting side effects including disturbed gastrointestinal motility, bradycardia, nausea and vomiting.
The development of selective M4 positive allosteric modulators (PAMs) is a strategy to overcome the challenges of developing selective orthosteric muscarinic agonists. Indeed, studies with M4 PAMs have shown that selective activation of M4 mAChRs can reverse both hyperdopaminergic and hypoglutamatergic behaviors in preclinical models. Accordingly, the compounds of the present invention, which are allosteric modulators of the M4 muscarinic acetylcholine receptor, are believed to be useful in the treatment of Alzheimer's disease and other diseases mediated by the muscarinic M4 muscarinic acetylcholine receptor.